Development and assessment of a membrane bioreactor for wastewater treatment

Abstract

A woven fibre membrane unit was developed locally and used for this study. The membrane unit was submerged in the aerobic tank of an anoxic-aerobic biological treatment process to treat dairy effluent. Another unit similar to the process above was also set up but with a gravity settling tank replacing the membrane unit. Kinetic studies were done on both processes and their results compared. Effluent characteristics of both processes were also compared. For the membrane bioreactor, the effectiveness of two backflush methods ie air and water, were tested. In this study it was discovered that air backflush could remove the clogging formed at external and internal pores of the membrane better than water backflush measured in terms of rate of pressure recovery and energy consumption. The effluent quality of the membrane bioreactor (MBR) in terms of COD, nitrogen, suspended solids and turbidity was excellent under all conditions tested and was better than conventional activated sludge system. The COD removed based on the average value in MBR was 96.02 % compared to 94 % conventional system. A mass balance of the processes showed that 79 % in the form of nitrogen fed into both the MBR and conventional systems was denitrified. The effluent suspended solids from the MBR was not determinable while that of conventional system was 2.09 mglL. Maximum specific nitrification rate determined graphically was 3.0 d-I and 2.10 d-I in conventional and MBR respectively. The smaller value of nitrification rate in MBR could be attributed to large number of nitrifier volatile suspended solids in the system. The estimated true yield and decay rate coefficients of conventional system is 0.204 kgVSS/kgCOD and 0.013 d-I respectively. The flux obtained from this study is between 22 and 92 L/m2.h. However, the average flux value is 57 L/m2.h for the whole period of operation. Regular flux decline observed during operation was improved through regular backwash with air and chemical cleaning. The transmembrane pressure did not as whole increase more than 55 kPa due to frequent cleaning of the membrane surface.